1. Field of the Invention
This invention pertains to methods of evaluating optical scratches and other damage to coated and uncoated optical samples. In particular, the present invention pertains to an apparatus and method permitting detailed scanning throughout the depth of an optical sample to identify the exact location of imperfections within the sample.
2. Description of the Prior Art
Previous techniques for evaluating the quality of optical samples have included:
a. Straight through or bright field technique which does not discern subtle contrasts, PA1 b. Normarski or back reflection technique which does not look at the subsurface but only the surface scratches, and PA1 c. the dark field or grazing angle reflection technique. None of these techniques provide information on the size of small imperfections beneath 500 angstroms or the position of such imperfections.
Previous internal microscopy has been limited to thin plates with beveled edges. This technique creates numerous reflections within the plate. Since each reflection must lose some light, this method is limited to loss spectroscopy.